Metallocene and a new single site catalyst component

ABSTRACT

The present invention discloses a catalyst system based on a metallocene catalyst component and a new single site catalyst component for the production in a single reactor of improved polyolefins having a bimodal molecular weight distribution.

CROSS-REFENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/433,025, now issued as U.S. Pat. No. 8,455,386, filed on Mar. 28,2012, which is a Continuation of U.S. patent application Ser. No.11/665,716, filed on Apr. 18, 2007, now issued as U.S. Pat. No.8,178,634, which is a National Stage entry of PCT/EP05/55398, filed onOct. 20, 2005, which claims priority from European Application No.04105218.4, filed on Oct. 21, 2004.

The present invention relates to a process for the production of anolefin polymer, in particular a bimodal polymer of ethylene, using twodifferent new single site catalyst components. It also relates topolymers produced by the process. The invention also relates to use of acatalyst to form polymers of the present invention. The catalyst isadvantageous, since it facilitates performance of the process in asingle reactor, producing a multimodal product having a highlycrystalline, low molecular weight component, and a further componenthaving low density. The catalyst system of the present invention can beused in gas phase, slurry, multizone circulating reactors (MZCR) andsolution processes

In many applications in which polyolefins are employed, it is desirablethat the polyolefin used has good mechanical properties. It is knownthat, in general, high molecular weight polyolefins have good meltstrength and good mechanical properties. Additionally, since thepolyolefin must usually undergo some form of processing, such asinjection or blow moulding processes, and extrusion processes and thelike, to form the final product, it is also desirable that thepolyolefin used has easy processing properties. However, unlike themechanical properties of the polyolefin, its processing properties tendto improve as its molecular weight decreases, particularly in theabsence of any long chain branching.

Imparting potentially antagonistic properties such as easy processingand high melt strength, rigidity and toughness to high performancepolyolefin resins has always posed serious challenges to scientists andengineers. It requires quite ingenious catalyst formulations andprocess-engineering design. Modern multi reactor, cascade processtechnologies employing state of the art catalysts have been used in anattempt to overcome the challenge. By judiciously controlling theconditions in each reactor, such as precise control of reactantconcentrations, catalyst's residence time, and polymer block ratios,they enable sequential and in-situ production of polymer blends withintermingled chains of distinctly different lengths and compositions. Inthese so-called bimodal resins a compromise of counteracting propertiesmay be reached.

Cascade reactor technologies comprise in majority two stirred tankslurry reactors, two slurry loop reactors or two gas phase reactors inseries. Processes also exist wherein a combination of a loop and afluidised bed gas phase reactor is employed. Reactor design,configuration, and conditions governing different cascade processes arequite different and may vary substantially from process to process. Theyall have however one distinct feature: they ensure, in one of the inseries configured reactors, the production of a high density lowmolecular mass polymer component and in the other reactor, theproduction of a high molecular mass low density polymer fraction. Majorchallenges in all varieties of cascade technologies are:

-   -   1. Frictionless, unidirectional transfer of monomer, diluents        and product from the first reactor to the second reactor, to        avoid reactors cross contamination.    -   2. Perfectly synchronized catalysts life and residence times to        ensure intimate polymer intermixing and homogenisation.

Additionally it is desired that the final bimodal exhibit well definedmelt flow and density.

Each cascade technology has its own specificity.

-   -   The stirred tank slurry process employs gaseous monomer,        ethylene with hexane as the preferred solvent as disclosed for        example in Boehm (J. Appl. Polym. Sci., 22, 279, 1984). Along        with catalysts and co-catalyst hydrogen is fed into the first        reactor to reduce the molecular mass in the first stage and        butene is introduced into the second reactor to lower the        density. The stirred tank technologies have simplified reactor        design and are easy to operate. The low monomer partial pressure        and long residence times require, however, very high catalysts        activities and life times.    -   In two slurry loop cascade processes, ethylene with a        combination of butene/hexane or hexene/isobutane as        co-monomer/solvent pair can be used. The reactors residence        times are shorter and catalysts with moderate activities are        tolerated. The major challenge in this type of processes is to        prevent excess H₂ or co-monomer to enter into the next reactor.    -   A slurry loop reactor may be combined with a fluidised bed gas        phase reactor as disclosed by Borealis. The first stage, loop        reactor, insures rapid start up of the production. It uses        propane in supercritical phase as diluent with the advantage of        introducing a large quantity of hydrogen for the production of        low molecular mass fraction without the risk of H₂ bubble        formation and reactor pressure instability. Additionally,        polymer dissolution and reactor fouling issues are eliminated        due to low solubility in propane whose critical temperature        remains below polymer's melting point. The second stage, the gas        phase reactor provides good density regulation and excellent        product flexibility.

Ziegler-Natta catalysts are predominantly used in cascade technologies.They fulfil conditions, such as moderate to high activities, goodhydrogen response and co-monomer incorporation capability imposed by thecascade process. Their good thermal and chemical stability guaranteesthat they survive the relatively long overall residence times of thereactors. They produce however short polymer chains in the highmolecular weight low-density fraction that remain in the amorphousphase, and do not contribute to tie molecules formation. Additionally,the branch rich, non-crystallisable low molecular weight materialgenerally leads to de-mixing and phase separation and is not favourableto mechanical properties.

Single site catalysts in general and metallocenes in particular areideally suited to be used in cascade technologies for the production ofboth fractions of bimodal polyethylene. Selected bridged metallocenecatalysts with excellent hydrogen response and co-monomer incorporationcapabilities allow the easier production of the bimodal polyethylenewithout excessive use of hydrogen and co-monomer, and therefore withlittle or no risk of the second reactor contamination. Their applicationis particularly advantageous since their narrow disperse polymers,permit precise design of the composition of each fraction particularlythat of the low density, high molecular mass fraction. In metallocenebased copolymer the branches are statistically distributed and are veryeffective in assisting tie molecule formation and preventing chainslongitudinal diffusion and lateral slippage.

In cascade processes, polymer particle formation starts with catalystparticles being gradually fragmented by infused layers of high densityand low-density polymer fractions in tandem reactors to finally becomepolymer particles. The solid-state morphology of the resultingpolyethylene is that of a biphasic polymer alloy, in which thehigh-density, homo-polymer component acts as the matrix for thelow-density copolymer part as can be seen in FIG. 1. The high molecularmass copolymer chains traverse several crystalline and amorphous layersand interconnect adjacent crystalline lamellae as tie molecules. Tiemolecules density is directly related to the chain length, molecularweight distribution (MWD), number and type of the side branches and thesemi-crystalline morphology for a given lamellar thickness. Theydetermine material's long- and short-term resistance to environmentaland/or mechanical stress. In bimodal systems, the crystalline domaindefines low strain rate of semi-crystalline polymers such as modulus,yield stress and slow crack growth properties whereas the amorphousregion determines the high strain properties such as impact, tear andfracture resistance. The concentration of the tie molecules determinesboth the low and high strain rate behaviour. A high concentration of tiemolecules can prevent or stop for example the brittle failure that isoccasionally initiated, even at low stress, by a small crack andformation of a crazing zone. The crazing zone is formed by highlyoriented fibrils under the applied stress concentration and ispostulated to be due to disentanglement of tie molecules connecting themicro crystallites and fracture of the fibril. The resistance tofracture is thought to improve by incorporation of various types ofbranches the long branches being more effective. Branches arepredominantly concentrated on tie-molecules that resist the chain pullout through the formation of micro-fibrils: they impede slow crackgrowth by reducing lamellar thickness and by decreasing thesusceptibility to craze initiation and development. Branches also serveto pin-down tie molecules, which are a priori less mobile than theirlinear counterparts. The pinning of branches at the crystal fold surfaceand represented in FIG. 2 is thought to be responsible for the very highfracture toughness of low density polyethylene (LDPE). High fracturetoughness is achieved in the longest branched chains that formtie-molecules. It is also equally important to optimise fracturetoughness by regularly spacing the branches, as for example inlow-density polyethylene produced with metallocene. The inter branchspacing sets the upper boundary to the effective molecular weight fortie-molecules.

The only disadvantage of using single site catalysts in cascadeprocesses to produce bimodal polyolefin is the difficult homogenisedextrusion due to very narrow molecular weight distribution of the twopolymer fractions and to the lack of overlapping consisting of chainswith intermediate molecular weight distribution. This is caused bynon-ideal residence time distribution which is independent of thecatalytic system in use, and cause some catalyst grains to leave thefirst reactor without any polymerisation and some catalyst grains tostay too long to be solely covered with a single fraction. The onlyremedy for this problem is the use of dual (or multiple) site catalystsin a single reactor. In this case each catalyst particle will have thecompositional ratio of the low and high molecular weight fraction whichwas pre-designed by the composition of its individual active sites. Theclose proximity of the two fractions makes the homogeneous extrusionpossible.

Production of polyolefin with a bimodal MWD in a single reactor has longbeen a goal of the polyolefin industry because single reactorconfigurations are significantly cheaper to build, have improvedoperability, and enable quicker product transitions than multi-reactorconfigurations. A single reactor can also be used to produce a broaderrange of products than can a set of cascaded reactors. Producing a resinhaving a bimodal MWD in a single reactor requires however highlysophisticated catalytic systems with at least two very different activesite populations. It was thought that metallocenes, with their vaststructural diversity, could provide highly chemo-selective active sitestructures with distinctly different hydrogen and co-monomer responseand thereby provide a facile route to dual site catalysts. The area ofdual site single reactor systems has been the object of a lot of effortsby the present applicant such as disclosed for example in U.S. Pat. No.6,255,428, EP-A-0790259, U.S. Pat. No. 6,410,476, WO/03029302, U.S. Pat.Nos. 6,380,311, 5,914,289, EP-A-830395, or U.S. Pat. No. 5,719,241. Themethod however still needs much improvement.

LIST OF FIGURES

FIG. 1 represents the molecular weight distribution of a bimodalpolyethylene resin and its relation to the dispersion of the low densityfraction, represented by the dark areas, in the high density matrix,represented by the light areas.

FIG. 2 is a schematic representation of semi-crystalline polyethylene.

FIG. 3 represents the log/log curve of stress as a function of time fora bimodal polyethylene, showing the transition between ductile andbrittle behaviours.

Many applications still require improved polyolefins and there is stilla need to control the molecular weight distribution of the polyolefinproducts more closely, so that the miscibility of the polyolefincomponents can be improved, and thus the mechanical and processingproperties of the polyolefins. In particular it is desirable to improvethe catalyst system employed in the manufacture of multimodalpolyolefins, in order to achieve these improvements in the polymersthemselves.

It is an object of the present invention to solve the problemsassociated with the above prior art by providing an improved catalystsystem.

It is also an object of the present invention to provide an improvedmethod for the formation of olefin polymers, employing new catalystsystems.

It is a further object of the present invention to develop a method thatis capable of forming multimodal, particularly bimodal, olefin polymers,preferably in a single reactor.

Accordingly, the present invention provides a catalyst composition forthe production of a multimodal olefin polymer, that comprises:

-   a) metallocene catalyst components of general formula (I):    R″(CpR_(m))(C′pR′_(n))MQ₂  (I)    wherein Cp is a substituted or unsubstituted cyclopentadienyl ring;    C′p is a substituted or unsubstituted cyclopentadienyl ring R″ is a    structural bridge between Cp and C′p imparting stereorigidity to the    component; each R or R′ is the same or different and is selected    from a hydrocarbyl group having from 1-20 carbon atoms, a halogen,    an alkoxy group, an alkoxyalkyl group, an alkylamino group or an    alkylsilylo group and two neighbouring substituents can be linked to    form a cycle; M is a metal atom from Group 4 of the Periodic Table;    and each Q is a hydrocarbon having from 1-20 carbon atoms or is a    halogen    or of formula (I′)    R″(CpR_(m))XMQ₂  (I′)    wherein R″, Cp, M, Q and R_(m) are as defined previously and X is an    heteroatom, substituted or unsubstituted and selected from Group 15    or 16 of the periodic table, preferably, N, P or O-   b) and a single site catalyst component of formula (II):    (L)_(n)Me(Q′)_(p)  (II)    wherein L is an heteroatom-containing ligand; n is an integer of 1,    2, or 3; Me is selected from Ti, Zr, Sc, V, Cr, Fe, Co, Ni, Pd, or a    lanthanide metal; each Q′ is independently a hydrocarbon having 1-20    carbon atoms or a halogen; and p is the valence of Me minus the sum    of the coordination numbers of all L.

An active catalyst system is prepared by adding an appropriateactivating agent.

The catalyst system of the present invention is used in the homo- orco-polymerisation of alpha-olefins. The polymers obtained with thecatalyst system of the present invention have a bimodal or multimodalmolecular weight distribution. The molecular weight distribution isdefined by the polydispersity index that is the ratio Mw/Mn of theweight average molecular weight Mw over the number average molecularweight Mn.

In a preferred embodiment according to the present invention, (CpR_(m))is a substituted or unsubstituted cyclopentadienyl and (CpR′_(n)) is asubstituted or unsubstituted fluorenyl.

In another more preferred embodiment according to the present invention,the metallocene component has Cs symmetry. More preferably, (CpR_(m)) isan unsubstituted cyclopentadienyl and (CpR′_(n)) is a fluorenylsubstituted in positions 3 and 6. Preferably, the substituents on thefluorenyl are the same and they are tert-butyl.

In a further more preferred embodiment according to the presentinvention, the metallocene component(s) is (are) bridged bis-indenylstructure(s), preferably substituted in positions 2 and/or 4, or it is abis-benzindenyl component and the cyclopentadienyl can be substituted atthe distal and/or proximal positions.

Bis-cyclopentadienyl catalyst components can also be used.

The preferred metal M is hafnium.

In yet another preferred embodiment, the metallocene component is aconstrained geometry zirconocene or titanocene, more preferably it is aconstrained geometry titanocene.

The metallocene component is responsible for the high molecular weightcomponent of the polyolefin, and hafnium is the preferred metal becauseit has excellent comonomer incorporation and inherently produces highmolecular weight chains. The constrained geometry titanocene alsoproduces high molecular weight chains.

Any activating agent having an ionising action known in the art may beused for activating the metallocene component. For example, it can beselected from aluminium-containing or boron-containing compounds. Thealuminium-containing compounds comprise aluminoxane, alkyl aluminiumand/or Lewis acid.

The aluminoxanes are well known and preferably comprise oligomericlinear and/or cyclic alkyl aluminoxanes represented by the formula:

for oligomeric, linear aluminoxanes and

for oligomeric, cyclic aluminoxane,wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R isa C₁-C₈ alkyl group and preferably methyl.

Suitable boron-containing activating agents that can be used comprise atriphenylcarbenium boronate such astetrakis-pentafluorophenyl-borato-triphenylcarbenium as described inEP-A-0427696, or those of the general formula [L′-H]+[B Ar₁ Ar₂ X₃ X₄]—as described in EP-A-0277004 (page 6, line 30 to page 7, line 7).

Aluminoxane activating agents are known to have a detrimental actionwhen the metal used in the metallocene component is hafnium. It isbelieved that the presence of trimethylaluminium (TMA) inmethylaluminoxane (MAO) is responsible for the low activity of thehafnium-based metallocene catalyst systems by producing a non-activebimetallic stable intermediate. TMA free modified MAO should be used(MMAO).

If a sterically hindered Lewis base is added to MAO, TMA can be trappedand thus be prevented from interacting with the cationic species. Asterically hindered organic Lewis base is an organic compound containingat least one atom having at least one free electron pair and in whichthis atom or these atoms has or have at least one directly adjacentgroup which largely shields it and which can cause steric hindrance. Themost preferred sterically hindered Lewis base is a bulky phenol.

The aluminoxane and Lewis base are mixed together and left to react fora period of time of from 30 minutes to 2 hours, preferably about onehour in order to reach equilibrium.

Alternatively an activating agent based on borate can be used when metalM is hafnium.

In a preferred embodiment according to the present invention, metal Mein the single site component (L)_(n)Me(Q′)_(p) is preferably Fe.

The preferred single site component according to the present inventionis a compound of formula V

wherein R are the same and are alkyl groups having from 1 to 20 carbonatoms, preferably 1 to 6 carbon atoms and more preferably methyl groups,R′ and R″ are the same or different and are alkyl groups having from 1 à20 carbon atoms substituted or unsubstituted or aryl groups having from1 to 20 carbon atoms substituted or unsubstituted.

Preferably, R′ and R″ are the same or different and are selected from asubstituted or unsubstituted alkyl having from 1 to 6 carbon atoms orare a unsubstituted or substituted aryl. The aryls group can themselvesbe substituted with alkyls having from 1 to 6 carbon atoms or withsubstituted or unsubstituted aryls having from 1 to 6 carbon atoms. Morepreferably, R′ and R″ are different and comprise substituted phenyls.Most preferably, R′ is a phenyl substituted with methyls in positions 2and 6 and R″ is an anyline substituted by two phenyls, each beingsubstituted by a tert-butyl.

The steric environment of the iron-based complex(es) is determined bythe substituents at positions 2 and 6 and optionally at positions 3, 4and 5 on the phenyls.

For the steric effect, the preferred substituents on the phenyls, ifpresent, can be selected from tert-butyl, isopropyl, methyl or phenyl.The most preferred substituents are methyls respectively at positions 2and 6 for R′ and phenyls respectively a positions 2 and 6 for R″, eachphenyl being substituted with a tert-butyl.

Preferably, the monomer used in the polymerisation reaction is ethylene.

When iron-based single site complex(es) are used in the copolymerisationof ethylene with a comonomer, it is observed that the comonomer is notincorporated in the backbone of the polymer chain. A highly crystallineethylene polymer without branches is obtained. The molecular weight ofthe polymer is however reduced and it is believed that the comonomeracts as a chain transfer agent. The comonomer is an alpha-olefin havingfrom 3 to 8 carbon atoms, preferably it is hexene and is used both formolecular weight regulation for high density polymer fraction anddensity regulation of the high molecular weight polymer fraction.

The combination of a hafnium-based metallocene catalyst component and ofan iron-based new single site catalyst component is thus particularlybeneficial in the production of bimodal polyethylene.

-   -   The hafnium-based component(s), responsible for the high        molecular weight fraction of the polymer, is very efficient in        comonomer incorporation and produces a low density high        molecular weight component.    -   The iron-based single site component(s), responsible for the low        molecular weight fraction of the polymer in the presence of a        comonomer does not require the presence of hydrogen to terminate        the chains.

The hafnium-based component can be replaced by a constrained geometrytitanocene.

Increasing the commoner concentration thus simultaneously reduces thedensity of the high molecular weight fraction and reduces the molecularweight of the low molecular weight fraction.

Instead of introducing the comonomer in the reaction zone, anoligomerisation catalyst system may be added for preparing the comonomerin situ. Oligomerisation catalysts systems are known by the man skilledin the art. Among the preferred oligomerisation catalyst system, one cancite for example the octanuclear nickel complex disclosed ininternational patent application PCT/EP2004/002145.

The invention also discloses a method for preparing a catalyst systemthat comprises the steps of:

-   a) providing first metallocene-based catalyst components) of formula    I    R″(CpR_(m))(C′pR′_(n))MQ₂  (I)    wherein Cp is a substituted or unsubstituted cyclopentadienyl ring;    C′p is a substituted or unsubstituted cyclopentadienyl ring R″ is a    structural bridge between Cp and C′p imparting stereorigidity to the    component; each R or R′ is the same or different and is selected    from a hydrocarbyl group having from 1-20 carbon atoms, a halogen,    an alkoxy group, an alkoxyalkyl group, an alkylamino group or an    alkylsilylo group and two neighbouring substituents can be linked to    form a cycle; M is a metal atom from group 4 of the Periodic Table;    and each Q is a hydrocarbon having from 1-20 carbon atoms or is a    halogen or of formula (I′)    R″(CpR_(m))XMQ₂  (I′)    wherein R″, Cp, M, Q and R_(m) are as defined previously and X is an    heteroatom, substituted or unsubstituted and selected from Group 15    or 16 of the periodic table, preferably, N, P or O-   b) and providing single site catalyst component(s) of formula (II):    (L)_(n)Me(Q′)_(p)  (II)    wherein L is an heteroatom-containing ligand; n is an integer of 1,    2, or 3; Me is selected from Ti, Zr, Sc, V, Cr, Fe, Co, Ni, Pd, or a    lanthanide metal; each Q′ is independently a hydrocarbon having 1-20    carbon atoms or a halogen; and p is the valence of Me minus the sum    of the coordination numbers of all L;-   c) optionally providing an oligomerisation catalyst system;-   d) providing an activating agent;-   e) optionally providing a co-catalyst;-   f) optionally providing a support;-   g) retrieving an active catalyst system.

The catalyst components are preferably supported on the same ordifferent supports.

The present invention also provides a process for producing a polyolefinhaving a multimodal molecular weight distribution, the process generallycomprising the steps of:

-   -   (a) polymerising an olefin monomer and optional comonomer in the        presence of a first catalyst, to form a first polyolefin        component; and    -   (b) polymerising the same olefin monomer and optional comonomer        in the presence of a second catalyst to form a second polyolefin        component.

In the method of the invention, the molecular weight distribution of thefirst polyolefin component overlaps with the molecular weightdistribution of the second polyolefin component, thus forming a polymerproduct that has at least a bimobal molecular weight distribution.

Thus, the present invention utilises at least two catalyst components(or two population of active sites) for producing at least two polymercomponents, each component forming part of the multimodal polymericproduct. It is preferred that at least two catalyst systems areemployed, and a bimodal or broad molecular weight distribution polymerproduct is produced. However, the invention is not limited to bimodalproducts only, and multimodal polymers may be produced if desired.

In a particularly preferred embodiment of the present method, thepolymerising steps (a) and (b) take place in a single reaction zone,under polymerising conditions in which the catalysts producing thepolymer components are simultaneously active.

Many known procedures for forming multimodal polyolefins have employed adifferent reactor for forming each component. The methods of the presentinvention are particularly advantageous, since they allow for theproduction of improved olefin polymers from a single reactor. This isbecause the catalysts employed in the present invention are moreeffective than known catalysts, particularly when utilisedsimultaneously in the same reactor. This has two distinct advantages.Firstly, since only a single reactor is required, production costs arereduced. Secondly, since the components are all formed simultaneously,they are much more homogeneously blended than when produced separately.

Although polymerisation in a single reactor is particularly preferred,the catalysts employed in the present invention are still particularlyeffective in producing the required polyolefin components of amultimodal product even when these components are produced in separatereactors. Accordingly, in some embodiments, separate reactors may beemployed for forming some or all of the components, if desired.

Although the invention may be applied to any olefin polymerisation, theolefin monomer employed typically comprises ethylene and/or propylene.Bimodal or multimodal polyethylene is the most preferred product.

The catalyst systems employed in the present invention may be employedin any type of co-polymerisation method, provided that the requiredcatalytic activity is not impaired. In a preferred embodiment of thepresent invention, the catalyst system is employed in a slurry process,which is heterogeneous. Preferred supports include a porous solidsupport such as talc, inorganic oxides and resinous support materialssuch as polyolefin. Preferably, the support material is an inorganicoxide in its finely divided form.

Suitable inorganic oxide materials are well known in the art.Preferably, the support is a silica support having a surface area offrom 200-700 m²/g and a pore volume of from 0.5-3 ml/g.

The amount of activating agent and metallocene usefully employed in thepreparation of the solid support catalyst can vary over a wide range anddepend upon the nature of the activating agent.

The order of addition of the catalyst components and activating agent tothe support material can vary. In accordance with a preferred embodimentof the present invention activating agent dissolved in a suitable inerthydrocarbon solvent is added to the support material slurried in thesame or other suitable hydrocarbon liquid and thereafter a mixture ofthe catalyst components is added to the slurry.

Preferred solvents include mineral oils and the various hydrocarbonswhich are liquid at reaction temperature and which do not react with theindividual ingredients. Preferably, the support material is slurried intoluene and the catalyst components and activating agent are dissolvedin toluene prior to addition to the support material.

The present invention also provides an olefin polymer, obtainableaccording to a method as defined above. The most preferred polymerobtainable according to the present invention is high densitypolyethylene (HDPE).

Also provided is the use of a dual catalyst system for producing anolefin polymer.

Resins having a bimodal molecular weight distribution can be used inhigh density, blown film, application where they offer an attractivecombination of rheological properties in terms of shear response, lowdie swell, and high melt strength and of physico-mechanical propertiessuch as clarity/low gel, tear strength, Environmental Stress CrackResistance (ESCR). They further offer a good compromise of stiffness andimpact resistance. These high-density polyethylene (HDPE) films are easyto process and allow down gauging and cost saving measures.

Blow moulding grades have excellent processing capabilities because oftheir low die swell and high melt strength. They also have goodmechanical properties in terms of stiffness and ESCR. The containersprepared with the resins of the present invention can thus have thinwalls, thereby requiring less material, and yet evince the resins havingthe best combination of top-load and ESCR.

Pressure pipe for natural gas and drinking water distribution, isanother fast growing application for bimodal HDPE. The performancecriteria for pressure pipes are processability during the extrusionthrough annular dies as well as short and long term performanceproperties requiring resistance to environmental (chemical andmechanical) stress ESCR, Slow Crack Growth (SCG), and Rapid CrackPropagation (RCP). They must compete with incumbent materials such asconcrete and steel having long service lives of over 50 years.

High performance pipes produced with the bimodal resin according to thepresent invention have the ability to resist short and long term failuremechanism such as growth of an incidental crack in the pipe over longperiods of time under constant pressure (SCG), and resistance to RCP asa results of impact of a sharp object (impact failure). Additionally andmost importantly, they exhibit high creep rupture strength (highmodulus, high stiffness). The service lifetime is estimated viaLong-Term Hydrostatic Strength (LTHS) that is determined by MinimumRequired Stress (MRS) tests. These tests require a series ofpressure/failure time curves established at different temperatures witha number of pipes having prescribed length, diameter and wall thickness.Calculations and extrapolations are then carried out following themethod developed by Schulte (U. Schulte in 100 Jahre Lebensdauer;Kunststoffe, 87, p. 203, 1997.), and a Hooks stress/service time curveof well over 50 years at 20° C. is obtained. The curve, represented inFIG. 3, exhibits three distinct regions. After a short experiment time,a flat region in which at high stress the tested pipes failed in aductile mode. After a very long experiment time, another regioncharacterised by a very steep descent, during which the tested pipes,via a thermo-oxidative initiated change, failed in brittle mode. Inbetween, there is a less pronounced intermediate region characterized bya ductile/brittle transition “knee”. According to current results,metallocene based PE100 resin with ideal balance in fractional polymerproperties has service life times of over a century. The classificationPE100 is based on MRS at a temperature of 80° C., a pressure of 5 Mpafor a period of time of 1000 h according to the method of the ISO normTR 9080.

The invention claimed is:
 1. A method for preparing an active catalystsystem comprising: providing a metallocene catalyst component of:formula (I):R″(CpR_(m))(C′pR′_(n))MQ₂  (I) wherein Cp is a cyclopentadienyl ring;C′p is a fluorenyl ring; R″ is a structural bridge between Cp and C′p;each R or R′ is the same or different and is selected from a hydrocarbylgroup having from 1-20 carbon atoms, a halogen, an alkoxy group, analkoxyalkyl group, an alkylamino group or an alkylsilyl group or twoneighbouring substituents can be linked to form a cycle; M is a metalatom from group 4 of the Periodic Table; and each Q is a hydrocarbonhaving from 1-20 carbon atoms or is a halogen; m is an integer from 1 to4 and n is an integer from 1 to 8; or of formula (I′)R″(IndR_(q))₂MQ₂  (I′) wherein R″, M, Q and R are as described above,Ind is an indenyl group and q is an integer from 1 to 6; or of formula(I″)R″(CpR_(m))XMQ₂  (I″) wherein R″, Cp, M, Q, R and m are as definedpreviously and X is a heteroatom, substituted or unsubstituted selectedfrom P or O; wherein the metal is hafnium in the metallocene catalystcomponents of formula I or I′; and providing a single sitepolymerisation catalyst component of formula (II):(L)_(n)Me(Q′)_(p)  (II) wherein L is a heteroatom-containing ligand; nis an integer of 1, 2, or 3; Me is Fe; each Q′ is independently ahydrocarbon having 1-20 carbon atoms or a halogen; and p is the valenceof Fe minus the sum of the coordination numbers of all L; providing atleast one inorganic support; impregnating the inorganic support(s) withan activating agent comprising methylaluminoxane combined with asterically hindered Lewis base, wherein for formulas (I) and (I′), themethylaluminoxane is MMAO; adding a solution of the metallocene catalystcomponent to the impregnated inorganic support(s); adding a solution ofthe single site polymerisation catalyst component of formula (II) to theimpregnated inorganic support(s) or to another impregnated inorganicsupport; and retrieving the active catalyst system.
 2. The method ofclaim 1, wherein the metallocene component is of formula I and thefluorenyl ring is substituted in positions 3 and
 6. 3. The method ofclaim 2, wherein the substituents on the fluorenyl are the same and aretert-butyl.
 4. The method of claim 1, wherein the metallocene componentis a bisindenyl and the indenyl is substituted at positions 2 and/or 4and the cyclopentadienyl is optionally substituted at the distal and/orproximal positions relative to R″.
 5. The method of claim 1, wherein themetal is titanium in I′.
 6. The method of claim 1, wherein the singlesite catalyst component is represented by formula

wherein R are the same and are alkyl groups having from 1 to 6 carbonatoms, R′ and R″ are different and comprise substituted phenyls.
 7. Themethod of claim 1, wherein the sterically hindered Lewis base is a bulkyphenol.
 8. The method of claim 1, wherein the metallocene catalystcomponent and the single site polymerisation catalyst component offormula (II) are supported on the same impregnated inorganic support. 9.The method of claim 1, wherein the metallocene catalyst component andthe single site polymerisation catalyst component of formula (II) aresupported on different impregnated inorganic supports.